New cube linked to the old object, how to make them separate?
It looks like you added the little cube while you were in &quot;Edit&quot; mode, so it became part of the same mesh as the chair. You will need to go back into &quot;Edit&quot; mode, select all the faces of the little cube and delete them.

&quot;tab&quot; for edit mode
&quot;alt&quot;-a deselect all
&quot;3&quot; for face mode
select click a face on the little cube
&quot;Ctrl&quot;+l for all linked faces
&quot;p&quot; to separate to new item